The City of Philadelphia says a problem with their 911 system has been fixed.

"9-1-1 landline phone service has been restored. Residents calling 9-1-1 can use their landline phone. Verizon will remain making repairs to underground wiring, which is believed to have been the cause of the system interruption," the City of Philadelphia Office of Emergency Management tweeted Tuesday morning.

In an alert issued Monday afternoon, the city said landline calls to 911 were not reliable. Instead, residents were urged to use cell phones to contact 911.

The problem was primarily impacting West Philadelphia and Northwest Philadelphia.

Update: 9-1-1 landline phone service has been restored. Residents calling 9-1-1 can use their landline phone. Verizon will remain making repairs to underground wiring, which is believed to have been the cause of the system interruption.